1. Field of the Invention
The present invention relates to a near-field light generator for generating near-field light by using light propagating through a waveguide. And the present invention relates to a head used for thermally-assisted magnetic recording in which a magnetic recording medium is irradiated with near-field light, thereby anisotropic magnetic field of the medium is lowered, thus data can be written. Further, the present invention relates to a magnetic recording apparatus provided with the head.
2. Description of the Related Art
As the recording density of a magnetic recording apparatus becomes higher, as represented by a magnetic disk apparatus, further improvement has been required in the performance of a thin-film magnetic head and a magnetic recording medium. Especially, in the magnetic recording medium, it is necessary to decrease the size of magnetic microparticles that constitute the magnetic recording layer of the medium, and to reduce irregularity in the boundary of record bit in order to improve the recording density. However, the decrease in size of the magnetic microparticles raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume.
Recently, as a method for solving the problem of thermal stability, so-called a thermally-assisted magnetic recording technique is proposed. In the technique, a magnetic recording medium formed of a magnetic material with a large magnetic anisotropy energy KU is used so as to stabilize the magnetization; anisotropic magnetic field of the medium is reduced by applying heat to a portion of the medium, where data is to be written; just after that, writing is performed by applying write field to the heated portion.
A technique is well-known in which the heating of a portion to be written of the medium is performed by irradiating the portion with near-field light. For example, U.S. Pat. No. 6,768,556 and U.S. Pat. No. 6,649,894 disclose a technique in which a metal plate for generating near-field light, so-called a plasmon antenna, is provided on the opposed-to-medium surface, then near-field light is generated by irradiating the opposite side to the opposed-to-medium surface of the plasmon antenna with laser light guided through a waveguide.
One the other hand, the present inventors have devised a near-field light generator in which laser light propagating through a waveguide is coupled with a plasmon antenna in a surface plasmon mode to cause the excited surface plasmon to propagate to the opposed-to-medium surface, thereby providing near-field light, rather than directly applying the laser light to a plasmon antenna. The plasmon antenna in the near-field light generator is hereinafter referred to as a surface plasmon antenna. In the near-field light generator, the temperature of the surface plasmon antenna does not excessively rise because laser light is not directly applied to the surface plasmon antenna. As a result, there can be avoided such a situation in which the end, which reaches the opposed-to-medium surface, of a read head element for reading data signal or servo signal from the magnetic recording medium becomes relatively far apart from the magnetic recording medium due to the thermal expansion of the plasmon antenna, which makes it difficult to properly read the servo signal. In addition, there can also be avoided such a situation in which the light use efficiency of the near-field light generator is degraded because thermal disturbance of free electrons increases in the plasmon antenna. Here, the light use efficiency of a near-field light generator is given by IOUT/IIN (×100), where IIN is the intensity of laser light incident to the waveguide, and IOUT is the intensity of near-field light emitted from a near-field light generating end of the plasmon antenna after converting the laser light into surface plasmon in the plasmon antenna.
The above-described near-field light generator has a light that has propagated through the waveguide, however is not transformed into surface plasmon, thus is emitted from the end of the waveguide toward the magnetic recording medium. When performing a thermally-assisted magnetic recording, the emitted light must not cause unwanted writing and erasing, thus the write error must be avoided as much as possible. Especially, there must be avoided both situations in which a portion not to be heated of the magnetic recording medium is irradiated and heated with the emitted light just before applying write magnetic field, and in which the emitted light becomes a noise light by overlapping the generated near-field light. Here, it is very important to have a sufficient distance, on the opposed-to-medium surface of the head, between the emission position of the light emitted toward the magnetic recording medium and the generation position of the near-field light, or not to direct the light that has not been transformed into surface plasmon toward the magnetic recording medium.